The present invention relates to the field of Code Division Multiple Access (CDMA) communications with direct sequence/spread spectrum (DS/SS) modulated signals. It is concerned in particular with a novel adaptive signal receiver for CDMA communication systems.
A CDMA communication system transmits information from a number of users over a common channel using different code sequences referred to as signatures. In these systems, the transmitters send the information independently from each other. Therefore, signals from different users arrive asynchronously at the receiver. Because the cross correlations between the signatures for signals from different users are non zero, the received signal from a user is sensitive to nearby interfering signals from other users.
This sensitivity to interference from other users contrains the number of active users operating at a specified system bit error rate (BER), and therefore CDMA systems are said to bear a modest overall capacity in terms of number of users/bandwidth.
Some attention has already been focused on the Multiple Access Interference (MAI) and in the last decade there has been an explosion of interest on multiuser interference rejection and multiuser detection (MUD). The purpose of MUD is to achieve the joint demodulation of all user data streams. One of the major outputs of the extensive research activity, inspired by the discovery of MUD, was the recognition that the sensitivity to MAI of CDMA is not inherent to the access method itself, rather it is essentially related to the incorrect utilization, in a multiple-access environment, of the single-user correlation receiver that presently represents the optimum scheme for processing a single DC/SS signal in an additive white Gaussian noise (AWGN) channel (see xe2x80x9cOptimum Multiuser Asymptatic Efficiencyxe2x80x9d by S. Verdxc3xa, IEEE Transactions on Communications, September 1986, pp. 890-897).
The implementation of a multiuser receiver is not straightforward: firstly due to the high computational complexity for the optimum scheme and secondly because even sub-optimum lower-complexity schemes call for accurate estimation of the code timing, the carrier phase and/or the signal power of all the active network users. The latest step towards a practical implementation of a MAI-resilient detector is represented by the class of single-user adaptive receiver an example of which is disclosed for instance in xe2x80x9cMMSE Interference Suppression for Direct-Sequence Spread-Spectrum CDMAxe2x80x9d, by Upamanyu Madhow et Michael L. Honig, IEEE Transactions on Communications, Vol. 42, No. 12, December 1994, pp. 3178-3188. This type of receiver of capable of eliminating a substantial portion of the multiuser interference, but it needs to be adjusted by a proper training sequence that must be known a priori, before data transmission can occur.
A robust and simple receiver of this kind that can operate without need for a training sequence is disclosed in xe2x80x9cBlind Adaptive Multiuser Detectionxe2x80x9d, by Michael Honig, Upamanyu Madhow, Sergio Verdxc3xa, IEEE Transactions on Information Theory, Vol. 41, No. 4, July 1995, pp. 944-960. The simulation results presented therein are relative to the case of BPSK DS/SS synchronous signals with rectangular chips, ideal coherent detection and a low AWGN level. The ideal coherent detection is in fact a strong assumption that largely reduces the applicability of blind adaptive interference-mitigating detection (BAID) scheme to practical systems. The use of the BAID detector requires the prior knowledge of the useful channel carrier frequency and phase that is generally unknown to the receiver and that can not be accurately estimated prior to DS/SS signal detection.
In particular, the authors of said publication envisage application of BAID to the initial receiver acquisition phase only, just to bring the signal-to-noise ratio after detection in the vicinity of 0 dB.
In such conditions, a decision-aided detector can directly take over without the need of a special training sequence. Such decision-aided detector is disclosed in xe2x80x9cAdaptive Receiver Structures for Asynchronous CDMA Systemsxe2x80x9d, by Predag B. Rapajic and Branka S. Vucetic, IEEE Journal on Selected Areas in Communications, Vol. 12, No. 4, May 1994, pp. 685-697. Although optimal in performance, a data-aided detector requires the prior knowledge of the transmitted symbols. Assuming a slowly variant channel as compared to the symbol duration, it is possible to insert a preamble of known data in the data stream at regular intervals to help the detector convergence.
In this case, however, there is a penalty due to the increased data rate and the need for fast converging algorithms like Recursive Least Square that drastically increases the algorithm complexity and presents numerical stability problems. While the decision-aided approach does not require the prior knowledge of a training sequence, it has however a is major drawback in its poor performance under low signal-to-noise ratio conditions whereby symbol estimates become unreliable and detector performance rapidly degrades. This behavior is particularly harmful when the signal amplitude significantly fades, as is the case in many practical (mobile) systems or when path diversity is exploited.
In this case, equalization has to be performed on each Rake receiver finger at lower input signal-to-noise ratio. Furthermore, the adaptive detection approach in this known detector is not insensitive to the unknown phase of the useful signal and the useful channel carrier frequency error.
It is known that the data-aided MMSE algorithm provides intrinsic phase estimation on top of interference mitigation. This is an apparent advantage as it relates the algorithm convergence speed for interference mitigation to the carrier phase tracking. Over fading channels the algorithm performs poorly unless the useful signal phase is pre-corrected before entering the adaptive MMSE detector (see A. N. Barbosa, S. L. Miller xe2x80x9cAdaptive Detection of DS/CDMA Signals in Fading Channelsxe2x80x9d IEEE Trans. on Comm., Vol. 46, No. 1, January 1998 whereby a solution based on a complicated phase estimates to remove phase errors at the detector input is proposed). The proposed solution being phase transparent does not suffer from this important problem and allows to effectively separate the useful signal phase estimation from CDMA interference mitigation.
The present invention deals with an extension of the blind adaptive interference mitigating detection (BAID) scheme to the general case of asynchronous CDMA signals with arbitrary interferer timing and frequency offset in order to enhance the performance of a DS-CDMA receiver with affordable complexity increase.
It is an object of this invention to provide a blind adaptive receiver for CDMA signals which minimises the detrimental effect of multiple access interference (MAI) on the bit-error rate (BER) performance and which does not require using a known training sequence.
A further object of this invention is to provide a low-complexity blind adaptive receiver for CDMA signals that is robust to asynchronous MAI.
Yet another object of this invention is to provide a blind adaptive receiver scheme that is robust to the interferers frequency error and is invariant to a possible carrier phase offset.
These and other objects of the invention are attained by an adaptive receiver for CDMA signals which comprises means arranged to perform the symbol detection in the incoming data stream using a user signature sequence comprised of a first fixed term and a second complex adaptive part having a predetermined length that extends over a number of samples. Means are provided to update automatically and at successive regular intervals encompassing the duration of one symbol or a plurality of symbols.
The second complex adaptive part is determined starting from the error signal of the carrier frequency measured on the signal obtained after detection.
The blind adaptive detector incorporated in the CDMA receiver of the invention can be implemented in modular form well suited for a compact ASIC digital implementation on a user terminal. The embodiment includes unobvious techniques to avoid the impact of a definite number of bits being used by DSP/ASIC to represent the signals.
The solutions proposed herein are particularly suited for application to satellite communication systems, but they can be adapted for use in terrestrial communication systems as well. Also, the invention can be adapted for use in communication systems having a variable transmission rate as proposed for the third-generation standards for wireless CDMA communication.
The main advantages of the invention can be summarized as follows:
1. There is no need for training sequences to help algorithm convergence nor knowledge of interferer parameters. The only parameters required are the useful channel signature sequence and chip timing information as for a conventional detector.
2. Robustness to asynchronous CDMA interference even for large interferer power unbalance and carrier frequency offsets.
3. The detector is insensitive to the unknown phase of the useful signal and can adopt conventional BPSK symbol rate phase estimators.
4. Increased robustness to the residual basehand useful channel carrier frequency error as compared to conventional data-aided Minimum Mean Square Equalizer (DA-MMSE).
5. System performance is very close to the optimum DA-MMSE linear detector in the SNR region of practical interest for a coded system.
6. The adaptive detector can operate at very low SNR typical of a coded system and supports more spectral efficient Dual-BPSK Spread Spectrum signal formats (D-BPSK/SS).
7. It is well suited for a compact ASIC digital implementation on a user terminal.
The invention can find practical applications in the following domains:
Fixed and mobile satellite communication networks.
CDMA-based positioning systems.
Terrestrial CDMA networks.